1. Field of the Invention
This invention relates to a method for formation of a deposited film useful for obtaining a functional film, particularly a semiconductive film which is useful for electronic devices such as semiconductor device, optical input sensor device for an optical image inputting apparatus, photosensitive device for electrophotography, etc.
2. Related Background Art
In the prior art, for formation of functional films, particularly crystalline semiconductor films, suitable film forming methods have been individually employed from the standpoint of desired physical characteristics, uses, etc.
For example, for formation of silicon deposited films such as of amorphous or polycrystalline, i.e. non-single crystalline, silicon which are optionally compensated for lone pair electrons with a compensating agent such as hydrogen atoms (H) or halogen atoms (X), etc., (hereinafter abbreviated as "NON-Si (H,X)", particularly "A-Si (H,X)" when indicating amorphous silicon and "poly-Si (H,X)" when indicating polycrystalline silicon) (the so-called microcrystalline silicon is included within the category of A-Si (H,X) as a matter of course), there have been employed the vacuum vapor deposition method, the plasma CVD method, the thermal CVD method, the reactive sputtering method, the ion plating method, the optical CVD method, etc. Generally, the plasma CVD method has been widely used and industrialized.
However, the reaction process in formation of a silicon deposited film according to the plasma CVD method which has been generalized in the prior art is considerably complicated as compared with the conventional CVD method, and its reaction mechanism is not well understood. Also, there exist a large number of parameters for formation of a deposited film such as substrate temperature, flow rate and flow rate ratio of gases to be introduced, pressure during formation, high frequency power, electrode structure, structure of a reaction vessel, speed of evacuation, plasma generating system, etc. By use of a combination of such a large number of parameters, plasma may sometimes become unstable, whereby marked deleterious influences were exerted frequently on a deposited film formed. Besides, parameters characteristic of film forming devices must be selected for each device and therefore under the present situation it has been difficult to generalize the production condition.
Also, in the case of the plasma CVD method, since plasma is directly generated by high frequency or microwave, etc., in a film forming space in which a substrate on which film is to be formed is placed, electrons or a number of ion species generated thereby may give damages to the film in the film forming process to cause lowering in film quality or non-uniformization of film quality. Moreover, the condition suitable for crystallization of a deposited film is restricted and therefore it has been deemed to be difficult to produce a polycrystalline deposited film with stable characteristics.
On the other hand, for formation of an epitaxial deposited film such as of silicon, germanium, group II-VI or Group III-V semiconductors, etc., there have been used the gas phase epitaxy and the liquid phase epitaxy as defined in a broad sense (generally speaking, the strict definition of epitaxy is to grow another single crystal on a single crystal, both having the same single crystal axes, but here epitaxy is interpreted in a broader sense and it is not limited to the growth onto a single crystal substrate).
The liquid phase epitaxy is a method for precipitating a semiconductor crystal on a substrate by dissolving a starting material for semiconductor at high temperature to a super-saturated state in a solvent metal which is molten to a liquid and cooling the solution. According to this method, since crystals are grown under a state closely approximating to thermal equilibrium among various epitaxy techniques, crystals with high perfectness can be obtained, but on the other hand, bulk productivity is poor and surface state is bad. For such reasons, in an optical device which requires an epitaxial layer which is thin and also uniform in thickness, problems are accompanied such as low yield in device production, or influences exerted on device characteristics, etc., and therefore this method is not frequently used.
On the other hand, the gas phase epitaxy has been attempted by physical methods such as the vacuum vapor deposition method, the sputtering method, etc., or chemical methods such as hydrogen reduction of a metal chloride or otherwise thermal pyrolysis of a metal organic compound or a metal hydride. Among them, the molecular beam epitaxy which is a kind of the vacuum vapor deposition method is a dry process under ultra-high vacuum, and therefore high purification and low temperature growth of crystals are possible, whereby there is the advantage that composition and concentration can be easily controlled to give a relatively flat deposited film. However, in addition to an enormous cost required for a film forming device, the surface defect density is great, and no effective method for controlling directionality of molecular beam has been developed, and also enlargement of area is difficult and bulk productivity is not so high. Due to such many problems, it has not been industrialized yet.
The hydrogen reduction method of a metal chloride or the thermal pyrolysis method of a metal organic compound or a metal hydride are generally called the halide CVD method, the hydride CVD method, MO-CVD method. For these methods, by the reason that a film forming device can be made with relative ease and also as the starting materials, i.e. metal chloride, metal hydrides and organic metals, those with high purities are now readily available, they have been studied widely at the present time and application for various devices has been investigated.
However, in these methods, it is required to heat a substrate to a high temperature at which reduction reaction or thermal pyrolysis reaction can occur and therefore the scope of substrate material to be selected is limited, and also contamination with impurities such as carbon or halogen, etc., is liable to occur if decomposition of starting material is insufficient, thus having the drawback that controllability of doping is poor. Also while, depending on the application use of a deposited film, it is desired to effect bulk production with reproducibility with full satisfaction in terms of enlargement of area, uniformization of film thickness as well as uniformness of film quality and yet at a high speed film formation, under the present situation no technique which enables bulk production with maintaining practical characteristics satisfying the above demands has been established yet.